A tilt sensor is used for measurement of a tilt angle of a measured object on which the tilt sensor is mounted, relative to a horizontal surface or a vertical axis of the object. On the other hand, an acceleration sensor and a seismoscope are used for measurement of acceleration received by an object on which the sensor is mounted. Since both types of sensors differ in an object to be measured, different sensors suitable for measurement of respective physical quantities are usually used selectively.
However, a liquid sensor using an electrically conductive liquid has conventionally been known as usable for measurement of both tilt angle and acceleration. The liquid sensor utilizes a principle that a liquid surface always keeps itself horizontal in a stationary state. When a container of the sensor containing the liquid therein tilts, an angle of the container relative to the liquid surface in the container is detected so that a tilt angle of the container is measured. Conversely, when a horizontal acceleration is applied to the sensor container disposed horizontally, the liquid surface is tilted in the container, so that a tilt angle of the liquid surface is detected, whereby applied acceleration is measured. In this specification, a sensor measuring a tilt angle or an acceleration using such a principle is referred to as “liquid sensor.”
A resistance type and a capacitance type are known as a method or system of detecting a tilt angle between a surface of liquid contained in a container and the container.
JP-A-2001-13160 discloses a technique pertaining to the resistance type, for example. In this sensor, as shown in the longitudinal section of FIG. 20, a suitable amount of electrically conductive liquid 102 is enclosed in a circular cylindrical metal container 101 having a closed end, and an opening is closed by a metal disc 103. One or two pairs of metal electrodes 104 extend through and are fixed in the disc 103 in an electrically insulated manner. When the container 101 tilts or a horizontal acceleration is applied to the container 101, an angle between the container 101 and a surface 105 of the liquid in the container is changed such that a contact area of the metal electrode 104 with the conductive liquid 102 is changed, whereupon an electrical resistance is changed between the metal container 101 and each metal electrode 104. Accordingly, changes in the resistance value are measured so that a tilt angle of the container 101 or a magnitude of acceleration is detected.
In the case of the resistance type liquid sensor, however, the metal electrodes are in direct contact with the conductive liquid (electrolytic solution) and accordingly, electric current for the measurement of resistance flows through a boundary therebetween. This results in a chemical change such as elution of metal composing the electrode or electrolysis of conductive liquid. As a result, there arises a problem that it is difficult to secure stability and reliability of the sensor for a long period of time.
On the other hand, JP-A-5-172571 discloses a technique pertaining to the capacitance type liquid sensor, for example. As shown in FIG. 21, a container 112 includes a cylindrical frame 110 disposed horizontally and made from a conductive material. The frame 110 has both open ends closed by insulating plates 111. The container 112 is filled with a conductive liquid 113 whose amount corresponds to substantially one half of capacity of the container and an insulating liquid 114 having a smaller specific gravity than the conductive liquid 113. Arc outer electrodes 115 formed by dividing outer surfaces of a semicircular or circular shape are provided on outer surfaces of the insulating plates 111 respectively. When the container 113 tilts, a capacitor constituted by the both end insulating plates 111 sandwiched between an arc outer electrode 115 and conductive liquid 113 changes a capacitance thereof. An amount of change is measured so that a tilt of the container is detected.
In the capacitance type sensor, however, a usual insulating plate is used as a dielectric forming the capacitor. Since the insulating plate constitutes a part of the container 112, its thickness cannot be reduced so much. Accordingly, since it is difficult to render the capacitance of the capacitor larger and the detectivity is low, there arises a problem that it is difficult to increase the detection accuracy.
Further, JP-A-11-118412 discloses a displacement signal generating device of the capacitance type. The device includes a single electrode substance and two electrodes of respective dielectric structural bodies having the electrodes such as a chemically treated aluminum foil, all the electrodes being impregnated in an electrolyte contained in a container. Electric elements are electrically connected between the single electrode substance and the electrodes of the dielectric structural bodies. When the container filled with the electrolyte or each of the electrodes of the dielectric structural bodies is displaced, a contact area of each of the electrodes of the dielectric structural bodies with the electrolyte is changed such that a capacitance between the single electrode substance and each of the electrodes of the dielectric structural bodies is changed. The change in the capacitance is measured, whereby displacement is detected.
In the capacitance type sensor, however, the chemically treated aluminum foil is used as the electrodes of the dielectric structural bodies. A surface of aluminum foil is anodized so that an aluminum oxide coat serving as a dielectric is formed thereon. Since the chemically treated aluminum coat has a problem in the stability, it is difficult to secure the stability and reliability of the sensor for a long period of time.